The argument essay on the GRE revised test, for which you have 30 minutes, requires you to analyze a statement and discuss how well (or how poorly) reasoned it is. You do not have a choice of which topic to write on. The argument essay is somewhat different in nature than the issue essay. The instructions are straight forward:
Discuss how well reasoned you find this argument
For example, consider the following argument essay topic:
Speed limits on our state's highways should be eliminated in order to increase our state's prosperity. Because greater speed means more efficient travel, commercial deliveries will be faster, increasing business profits. Elimination of speed limits will also make driving more attractive to motorists, so that more people will use the highways, providing more highway toll revenues for the state. At the same time, safety on our highways will not be affected: daytime speed limits were eliminated last year in the western states of our country, and no significant increase in the number of accidents in these states has been reported.
As the instructions imply, for the argument essay you will need to evaluate the statement that is presented. Luckily, many of the argument topics have flawed conclusions that are easily noticed and for which you can quickly respond and indicate any inconsistencies. For the above argument topic, for example, the conclusion is that speed limits on highways should be eliminated, and several reasons are given why the speed limits should be eliminated. To point out inconsistencies in the argument, you may choose to write about the fact that not all people wish to drive fast, and many people will avoid highways if the speed limits are removed. Also, you would want to point out that the study that is referred to may have been flawed. There is no information given about the study, and any responsible reader should be skeptical about such an unsupported statistic.
During the course of the argument essay you will need to demonstrate your ability to write well, but more importantly you need to demonstrate your ability to dissect an argument, which further gives evidence of your skill as a discerning reader. Just as in the issue essay, there is more than one "right" answer. What matters most is that you demonstrate your ability to intelligently respond to an argument. You need to show that you can critique the provided statement, and you'll need to point out specific details and fragments in the provided snipped of text that you think support your claims. It is these skills that the GRE test takers are looking for.